To obtain a modifying agent used for modifying an asphalt/ cement(AC), having a compatibility with the AC and capable of improving the practical performance of the asphalt concrete prepared by using the AC modified by using the same additive.

This modifying agent for the AC is a mixed composition of (1) a high molecular weight SBR (RH) having at least approximately 300,000 weight-average molecular weight (Mw) and (2) a low molecular weight SBR (RL) having approximately ≤280,000 Mw, and has an approximately (80/20)-(25/75) RH/RL ratio and at least 5% difference between the bonded styrene contents in the RH and RL. The composition has an approximately 50,000-150,000 number average-molecular weight and a light dispersion/light refraction ratio in 1.8-3.9 range. The modified AC prepared by using the above emulsion-polymerized SBR composition has a high level of strength, elongation, tenacity and toughness and the concrete containing the AC is proven to have a high resistance against depression, wheel truck and low temperature tearing.